1. Field of the Invention
This invention relates to well drilling fluid additives. More specifically, there is provided a mixture of components which, when added to drilling fluid or circulated through a borehole before casing is placed in the well, is effective in substantially reducing loss of fluid from the borehole.
2. Description of Related Art
The problems of reducing fluid loss from drilling wells have been recognized and addressed for decades. The generic causes of fluid loss from boreholes to the surrounding earth formations are well-known. They include: natural fractures in the rocks drilled, induced fractures when pressure in the drilling fluid exceeds fracturing stress of the earth, cavernous formations, and highly permeable formations. Unfortunately, the cause of fluid loss in drilling a particular well is not always known. Therefore, a variety of responses are often employed in attempts to control loss of fluid from a well. If the cause is believed to be natural or induced fractures or caverns, corrective action may begin by circulating into the well a pill or slug of larger particles at high concentration. This pill may contain a blend of granular, fibrous and flake materials with a particle size distribution believed to be large enough to form a bridge of material in the fracture or cavern. It is important that the bridge be within the formation and not on the surface of the wellbore where it can be dislodged by the drill pipe.
After a bridge is formed, it will still be necessary to form a seal of finer material on the bridge to reduce fluid loss from the wellbore to an acceptable level. If highly permeable formations are open to the wellbore, it is also necessary to form a seal of these formations to decrease fluid loss. Thus, a sealing composition for drilling fluids has a wide range of applicability.
To form a seal over a bridge that has been formed in a fracture or cavern or over a highly permeable formation, it has been found that a matting or caking effect should take place. A gradation of particle sizes, shapes and rigidity is beneficial in forming the seal, and there is an optimum blend to produce maximum fluid loss control with the seal. Obtaining a very effective seal will have the effects of reducing the occurrences of stuck drill pipe, reducing frictional drag between the drill pipe and the borehole wall, aiding in running casing or liners in the well, and possibly improving the quality of logs run in the well.
Both water-based and oil-based fluids are commonly used for drilling. The loss of fluid is usually more costly for oil-based fluids, because the base fluid is more expensive, but loss of fluid can be quite costly with water-based fluids also, because of the chemicals in the fluid.
Chemicals are added to drilling fluids for increased density, viscosity, and gel strength, for lower friction between the drill pipe and the borehole wall and for other purposes. The chemicals added for forming a barrier to flow on the borehole wall or in openings connected to the wall, called lost circulation materials, must be compatible with all the other functions to be performed by the drilling fluid and with all the chemicals added to produce the desired properties of the drilling fluid. In addition, the chemicals are preferably non-toxic and biodegradable.
A variety of naturally-occurring products have been used as lost circulation materials in the past. U.S. Pat. No. 4,619,772 discloses ground durum derived from the outer portion of the endosperm of the durum kernel. This material serves more as a viscosifier than as a fluid loss agent, but actually serves both purposes. U.S. Pat. No. 4,474,665 discloses the use of ground and sized cocoa bean shells, said to be a universal lost circulation controller. This product has not been widely accepted in industry.
The report "Lost Circulation in Geothermal Wells--Survey and Evaluation of Industry Experience," Report No. SAND81-7129, prepared for Sandia National Laboratories in 1981, surveys broad industry experience with lost circulation materials and provides a listing of materials used in the past. The report includes the results of an extensive literature survey. As discussed in the report (Table 3), a variety of wood fibers, cane fibers, organic fibers, nut hulls and seed hulls have been offered for use. Also, graded blends have been available.
Carbon particles are often added to drilling fluids for the purpose of reducing the frictional resistance of the drill pipe in contact with the borehole wall. The carbon particles embed on the cake or mat formed at the wall of the borehole over permeable formations drilled during drilling operations. It is believed that the carbon particles serve as a lubricant in the cake. This lower frictional resistance is very useful in preventing sticking of the drill pipe during drilling, especially where high permeability formations are in contact with the borehole. Therefore, the carbon particles enhance the benefits of the fluid loss additives.
There remains a need for a blend of materials which can function more effectively to reduce fluid loss from a borehole in a wide range of circumstances, with water-based or oil-based fluids, which is compatible with the other functions desired in a drilling fluid, and which is economical to use.